1. Field of the Invention
Aspects of the present invention relate to a fuel cell, and a method and an apparatus for controlling operation of a fuel cell system.
2. Description of the Related Art
A fuel cell is a low pollution, high efficiency power generating system, that can directly transform chemical energy from a fuel into electric energy. In particular, fuel cells can use alternative energy sources, such as, natural gas, methanol, and the like instead of petroleum, to generate electricity. Fuel cells have, therefore, attracted attention as next generation energy sources. A fuel cell can be classified as a phosphoric acid fuel cell (PAFC), a molten carbon fuel cell (MCFC), a solid oxide fuel cell (SOFC), a polymer electrolyte membrane fuel cell (PEMFC), an alkaline fuel cell (AFC), and the like, according to the type of electrolyte the fuel cell includes. All of these fuel cells operate on basically the same principle, but use different fuels, and have different operating temperatures, catalysts, electrolytes, and so on.
In a direct methanol fuel cell (DMFC), a polymer membrane, capable of conducting a hydrogen ion, is used as an electrolyte, and a methanol fuel is directly supplied to an anode. The DMFC does not need a fuel reformer and operates at a temperature of less than 100° C., making the DMFC particularly suitable for use in a portable or small fuel cell structure.
Generally, the output of a DMFC varies according to its system characteristics, such as, the temperature of a stack, the amount and the concentration of a supplied fuel, etc. Specifically, the concentration of the fuel is a system variable closely related to a voltage-current curve of the fuel cell. In other words, a DMFC has an optimum fuel concentration for producing a maximum efficiency. Accordingly, for a conventional DMFC to achieve a stable operation and a high efficiency, a concentration sensor is used to set an optimum fuel concentration, and the DMFC system is controlled to operate at the optimum fuel concentration. However, most concentration sensors exhibit a decreased sensitivity over time when measuring a fuel concentration. Therefore, the reliability of concentration sensors is reduced over time, and thus, it is difficult to achieve stable and continuous operations in conventional DMFC systems.
To solve the foregoing problems, there has been proposed a method of controlling operation of the fuel cell without using a concentration sensor. For example, Korean Patent First Publication No. 2004-93421, Nov. 5, 2004, Sony Co. Ltd., entitled “FUEL CELL AND METHOD OF OPERATING THE SAME,” discloses a method of measuring the power density of an upper power generating unit placed in an upper fuel stream and the power density of a lower power generating unit placed in a lower fuel stream; and adjusting the fuel concentration when the voltage of the upper power generating unit differs from that of the lower power generating unit, so that a concentration sensor is not needed.
However, the foregoing related art teaches a fuel cell stack that requires two power generating units, at the upper and lower fuel streams, making the fuel cell stack more difficult to manufacture, as compared with a conventional fuel cell stack requiring a single power generating unit. Further, the foregoing related art cannot be applied to a fuel cell stack having a single power generating unit.